Organic Prestressing - An Example of an Effector System

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Organic prestressing—an example of an effector system P. A. Ribeiro do Carmo Pacheco* University of Oporto, Portugal

* The author won second place of the fib 2001 Diploma to Younger Engineers, category Research (see report in fib-news, 2001, 2, No. 4). Following an invitation by the jury, this contribution summarises the achievements of his PhD thesis as presented during the Berlin Technical Activities Workshop on Tuesday 2 October 2001.

Notation The following symbols are used in this paper G, Q OPS nc Sci or t Da Dc Dt s  s

= dead loading; live loading = OPS system = number of active contractions i = control section (relevant fibre in control cross section) = instant t = activity margin = compression margin = time step = stress = stress increment

Summary The extraordinary efficiency of some structural solutions found in nature may help structural engineers in the development of new structural solutions, especially now that a remarkable technological evolution allows for sophisticated applications to be implemented. Commonly, the increase in resistance of a structural element is understood to imply either a different geometry of its cross-section or a different structural material. And that has to be done on a permanent basis. An effector system or ‘artificial muscle’ is a structural ele1464^4177  2002 Thomas Telford and fib

ment with the capacity of modifying the strength of a structure (by an adequate supply of energy), thus improving conveniently its performance. An effector system may be regarded as an active control system that is also a structural element, with applications extending to situations of quasi-static loading. Organic prestressing is an example of effector system that is feasible within the present technological capacities. In fact, it is none else than a prestressing system under on-line control, with the capacity to increase/decrease the prestressing forces introduced in the cables, thus improving significantly the prestressing effect. This paper presents, very synthetically, some of the main subjects developed in the PhD Thesis (with the same title and in Portuguese) submitted in 1999 to the Faculty of Engineering of the University of Oporto, Portugal.

Introduction New concepts of active structural control were developed at the end of last decade under the names of ‘parastressing’1 and of ‘effector systems’.2 Both involve active control systems3 where actuators are not external supplementary elements, rather are structural elements themselves. Freyssinet and Zetlin investigated these ideas some 40 years ago. Most probably, these two remarkable structural engineers did not proceed with their research because the technological context of their time was unhelpful. A useful example of effector system is provided by organic prestressing systems (OPS), which have been object of several numerical applications.2,4–7 A prototype is now on its first steps of execution, but its resilience is assured as OPS makes use of well-known technologies. OPS results in an ‘optimised’ prestressing, because permanent undesirable stresses are avoided and prestressing time-dependent losses are greatly reduced. Furthermore, OPS allows the design of lighter and more slender

structures with the same structural materials. These structural solutions do fit particularly well to situations of high ‘live-load/dead-load’ ratio. In this paper, a synthetic and general approach to structural solutions of bio-structures is presented. Also, a brief description of the muscular contraction is called to emphasize the concept of effector system. Then, the methodology to implement OPS is presented together with the mathematical expressions in the algorithms of an efficient control strategy. Finally, examples of applications are provided and main conclusions of this research are put forward.

Bio-structures An immense variety of structural solutions exist in the bio-structures world. Some are simple and others are quite sophisticated. All are sources of rewarding research. Structural engineers will certainly identify some well known structural elements. Although shapes may differ, structural objectives are the same. Nevertheless, simple calculations show that the ‘design criteria’ in bio-structures is quite different from those in Civil Engineering. Obviously, the ‘auto-repair’ capacity of living materials is a major feature of bio-structures. In Table 1, four bio-structural elements very similar to common structural elements are shown. Up to now no structural element in structural engineering has been able to play the role of a muscle in a bio-structure. True, there are some features in active control systems that resemble muscles, but the latter are structural elements themselves. A muscle is a structural element with a variable stiffness. That change of stiffness is achieved by supplying energy. Therefore, a muscle—or an effector system—can be regarded as a structural element that gets stiffness out of energy. In other words, a muscle is a string with variable stiffness (Figure 1). Since the beginning of the 20th century, researchers from different areas of knowledge have been able to identify many structural systems and structural features of bio-structures.8–14 The contributions of D’Arcy Thompson9 and Hildebrand13 are emphasized,

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energy. Hydraulic jacks and electromagnets are examples of energy transformers.

BONES

LIGAMENTOUS

ARTICULAR

TENDONS

MUSCLES

TIES AND CABLES

EFFECTOR SYSTEMS

CARTILAGE

STRUTS, COLUMNS, BEAMS AND SHELLS

CONNECTORS

BEARINGS

Table 1 Main structural elements in animal bio-structures6,8–10

Effector systems

~ Figure 1 Representation of an effector system—string of variable stiffness

but the contribution of structural engineers is fundamental if meaningful conclusions are to be drawn from that specific research area. In Table 2, some classical examples of structural systems are displayed, together with more complex systems where muscles play a structural function. Many ‘lessons’ can be learned from all these amazing structural systems. And it is quite obvious that muscles provide a ‘special’ prestressing, which avoids the undesirable stresses that are implied in conventional prestressing. And that ‘special’ prestressing is more efficient because it is variable, only acting when required.

Construction materials have always been taken as stable materials, with constant properties. Any sensibility to environmental changes is regarded as undesirable and variations of behaviour are taken as external actions.2 Some variations involving transfer of energy can, nevertheless, be dealt with in a different way. Also, since the elasticity modulus of all materials depend upon their energetic state, control and modification of the latter implies control and modification of the former. This leads to two trivial questions: How can it be done? What structural advantage can be taken out of it? In the case of sensory or adaptative materials, this is achieved by direct induction.3 Otherwise, energy transformers have to be used for an indirect induction. Energy transformers are to be taken as mechanisms introducing elastic energy into a structure out of other forms of

The best answer for the second question is in nature. Muscles are structural elements whose microscopic units are the sarcomers. These organic units are made of two kind of proteins: actin and miosin. When a contraction ‘decision’ is taken, a chemical energy induction takes place, providing a relative displacement of actin and miosin that changes the sarcomers configuration. This process alters the muscle elasticity modulus and modifies the stress state of the structure where the muscle is included. This ‘effector system’ ensures no undesirable stress states are generated in the bones, thus improving the structural performance of such a biomechanic structure. An effector system or ‘artificial muscle’ is a structural element with the capacity of modifying the strength of a structure (by adequate energy supply) improving conveniently its performance, typically whilst under specific actions. One possible answer to the first question is the OPS.

Energy Energetic State B

Energetic State A

Stiffness (A)

B

Stiffness (B)

A

~ Figure 2 Stiffness change by energy induction

~ Figure 3 Change of stiffness in muscles 108








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biceps braquial

P

P

umer

Table 2 Examples of structural systems in bio-structures3,5,11,15








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Organic prestressing

where

Organic prestressing is a stress-triggered self adjusting prestressing system. This system is based on well-known technology. The main elements are the organic anchorages, the tendons and the electronic circuit. All of them are widely used with reliable results. Obviously, the prestressing cables must be unbounded. Their design and construction technologies are similar to those required in post-tensioned unbounded prestressing structures, and the electronic circuit, which includes sensors, electric cables and electronic components, is very similar6 to a common active control system circuit. Organic anchorages are anchorages with servo-hydraulic systems incorporated. This means that the jacks stand between the anchorage and the structure. The control strategy is very simple.2,4,6 It is based on an algorithm quite similar to the classic ‘on-off’ algorithm. In short, if compression is low, OPS produces ‘contractions’ (prestressing forces are amplified), and if compression is high, OPS produces ‘descontractions’ (prestressing forces are reduced). In mathematical terms, this is stated by expressions (1) 8 OPS > Dai < sSci ðGÞ þ stSci ðQÞ þ nct  s Sci < Dci > > > > > > ) nctþDt ¼ nct > > > > > < sSci ðGÞ þ st ðQÞ þ nct  s OPS Sci Sci > Dci > > ) nctþDt ¼ nct þ 1 > > > > > t > sSci ðGÞ þ sSci ðQÞ þ nct  s OPS > Sci < Dai > > > : ) nctþDt ¼ nct  1 ð1Þ

sSci ðGÞ

stSci ðQÞ

OPS s Sci

nct and nctþDt

OPS nct  s Sci

Dci and Dai

is the stress at the relevant fibre in control cross section i due to dead loading; is the stress at the relevant fibre in control cross section i due to live loading at instant t; is the stress increment at the relevant fibre in control cross section i produced by one contraction; are the number of active contractions at instants t and t þ Dt. is the stress at the relevant fibre in control cross section i due to action of the organic prestressing at instant t; are the compression margin and the activity margin of the organic system; (these are the stress levels that make the sensors produce signals).

The generalisation of this algorithm to continuous beams is established in a similar manner.6 The delay of the response (both mechanical and electronic), as well as the consideration of any loading configuration, can be easily integrated into this methodology with no change in the fundamental logical procedures implicit in the mathematical expressions. This is explained with all detail in reference 6. Numerical analysis involves several aspects

H.D.P.E Tube

Grease

. calculation of prestressing losses taking into account the particular properties of organic prestressing; . definition of realistic evolutive loadings whose effects are at least equivalent to those defined in design codes; . analysis of control specific problems through adequate mathematical models; . analysis of uncertainties; . fatigue damage assessment and consideration of fretting fatigue; . ultimate and service limit states assessments (based on conventional procedures); . dynamic analysis including the control action dynamic effects; . definition of reliability procedures in design and in construction (emergence supplying units, redundant safety systems etc). Those issues are already studied,2,4–7 but testing by experimental analysis is required. That is the goal of the present stage of this research. The control effect produced by OPS may be understood in Figure 4, which refers to a loading case of the viaduct presented in Table 3, where eight OPS cables are implemented (two in each intermedium span and one in each extreme span). One of the most important features of OPS is the fact that the prestressing loses are greatly reduced. Because the hydraulic jacks are incorporated into the structure, they can compensate instantaneous losses. On the other hand, time dependent losses are relevant only in the permanent component of prestressing. In the example referred before, the difference found in two distinct solutions

Strand

~ Figure 4 Organic anchorage, unbounded tendon and typical layout of prestressing cable for a simply supported beam 110








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12.50

35. 00

32.50

32.50

37. 50

35. 00

Metro Tunnel Urban Viaducts with strong conditionalisms of slenderness and weight with OPS 4 30 m B B

A

View A-A

20 m

A Current road bridge with OPS

Cross-Section B-B 2

Precast concrete Silos designed with Snoko System with Organic Prestressing associated with locomotion functions

OPS

6

6,15

Zonaunder do tabuleiro a construir Part of the bridge deck construction 3.00

Ancoragem da viga ao troço do tabuleiro já construido. 7. 50 27. 00 Launching Gantry with OPS

49.50

9.00

57. 00

36.00 45.00

10,11

Table 3 Organic prestressing applications

developed for the viaduct, one with OPS and one other without OPS, is quite obvious. Another relevant aspect of OPS is related with the cross-section design of the structural elements. The value of the prestressing force in a prestressed structural element has to fall








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inside an interval implied by conditions expressing the design specifications. An empty interval requires an increase in the size of the crosssection or a new conception of the structure. That situation, which is relatively common in conventional prestressing design, does not

exist in organic prestressing design (or is extremely reduced), because prestressing forces are never too high. At the present stage of knowledge, the following balance of benefits/difficulties can be stated as follows

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(kPa)

(nc)

0

32,5

65

100

137,5

172,5 30

0

(m)

-2000

20

-4000 -6000

3 Figure 5 Stresses at bottom fibres and organic cables contractions under a three axle vehicle loading case7

10

-8000 -10000

0 σ(G)

σ(G+Q+OPS)

nc left cable

nc right cable

3 Figure 6 Total prestressing losses7

30% 20% 10% 0%

(m) 0

35

70

105

W ithout OPS

140 W ith OPS

(1) Advantages of OPS 50% reduction of prestressing losses;2,4,6 For slow loadings—until 70% reduction of stiffness;2,4,6 For slow loadings—until 30% reduction of structural mass;2,4,6 Lower permanent stresses;6 Lower deflections;6 Lower creep deformations;6 (2) Issues demanding special care Fatigue;4 Dynamic effects;6 Cost (powerful pumps for ‘fast’ loadings);6 Hyperactivity (control system);7 Instability (control system);7 Reliability.7 There are consistent procedures to overcome most of the difficulties,6,7 but for the proposed methodology, applications with ‘fast’ moving loadings require powerful pumps and may imply dynamic problems. Obviously, before further developments, and in the next steps of research, structures with ‘slow’ loadings will be considered first.

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Examples Several examples have been studied. In some of them, although structural advantages are recognized, difficulties (mentioned before) do exist (using only well-known technologies). In other cases, applications can be developed and implemented with success. Better results were found in structures with high ‘live load/dead load’ ratios and with relatively ‘slow’ loadings.5–7 Under the present technological capacities, one of the most promising applications is with launching gantries.

Conclusions The implementation of structural solutions of nature into engineering structures is a research field of immense interest. The modification of structural stiffness by the induction of energy is a subject that it is in its infancy, but it should be accepted that the concept of effector sys-

tem (or artificial muscle) opens new frontiers to the conceptual design of structures. Organic prestressing is one example which exhibits remarkable potentialities, specially when lightness and slenderness are envisaged. The theoretical fundaments of organic prestressing design are already developed and numerical results sustain its usefulness. The great reduction of permanent compressions and prestressing losses allow for a more rational use of prestressing. In structures with high live-load/dead-load ratios and with slow loading actions, organic prestressing can be a success, but experimental research is essential at this stage of knowledge and it is already being implemented.

References 1. Montens, S., A global concept for 21st century bridges: parastressing. Proceedings of the FIP Symposium on Post-Tensioned Concrete Structures, London, 1996, 739–746. 2. Pacheco, P. and Ada˜o da Fonseca, A., Effector systems in structures—conceptual design of structures. Proceedings of the IASS Symposium, Stuttgart, 1996, 339–346.








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3. Soong, T. T., Active Structural Control: Theory and Practice. Longman, London and Wiley, New York, 1990. 4. Pacheco, P., Ada˜o da Fonseca, A., Organically prestressed multi-span continuous box girders. New technologies in structural engineering. Proceedings of the IABSE International Conference, Lisboa, 1997, 527–534. 5. Pacheco, P., Quinaz, M. C. and Ada˜o da Fonseca, A., Applying organic prestressing on launching gantries. Proceedings of the Encontro de Estruturas Meta´licas e Mistas, Porto, 1997, 331–339 (in Portuguese). 6. Pacheco, P., Organic prestressing—an example of an effector system, PhD thesis, Department

of Civil Engineering, Faculdade de Engenharia da Universidade do Porto, 1999 (in Portuguese). 7. Pacheco, P. and Ada˜o da Fonseca A., Organic prestressing. Journal of Structural Engineering, ASCE, 2002, in progress. 8. Bombardelli, C., ‘Ossa lunghe: elementi naturali resistenti a flessione’, in ACCIAIO—‘Studi e ricerche’, 1982, pp. 408–415. 9. D’Arcy Thompson, On Form and Growth. Cambridge University Press, 1917. 10. Testut, T., Tratado de Anatomia Humana, Salvat Editores, S.A. Barcelona Buenos Aires, 1947. 11. Mcneill, A. R., Animal Mechanics, Sidgwick & Jackson London, 1968.

Obituary

Dr Max Birkenmaier

Max Birkenmaier 1915–2002 Born in Zurich, Max Birkenmaier started his career as a skilled carpenter, before he graduated at the ETH as a civil engineer in 1940. He was co-founder of Bureau BBR, together with his colleagues Brandestini and Ros, a study group to promote prestressing in Switzerland. They formed the Stahlton AG, which has since 1945 acted as a specialist company in the field








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of prestressing, using in practice the developments of BBR. Max Birkenmaier was Managing Director and later President of the board of this company. He was recognised as pioneer of prestressing and post-tensioning in Switzerland as well as abroad. Under his presidency, the first Swiss Standard for Concrete and Prestressed Concrete Structures was drafted and published in 1968. His reasoning power and profound thinking were also rated by the FIP and in 1966 Max Birkenmaier became the Swiss representative (Vice-president) on the Administrative Council, and a member of the Commission of Steels and Systems. Here he was responsible for the first issue of ‘Recommendations for acceptance and application of post-tensioning systems’ (1972). Following in

12. Fung, Y. C., Biomechanics—Mechanical Properties of 1981.

Living

Tissues.

Springer

Verlag,

13. Hildebrand, M., Analysis of Vertebrate Structure. John Wiley & Sons, Inc., 1988. 14. Pennycuick, C. J., Newton Rules Biology: A Physical Approach to Biological Problems. Oxford University Press, 1992. 15. Knauff, M. and Sadowski, A., Prestressing of circular tanks with tendons connected on the circumference—Snoko System, Proceedings of the FIP Symposium on Post-Tensioned Concrete Structures, London, 1996, 1014– 1021.

1978 the late Peter Misch, he was appointed as Senior Vice-president and member of the Praesidium, Max Birkenmaier’s outstanding contribution to the development of prestressed concrete and to the work of the FIP were recognised by the Honorary Doctorate of the Swiss Federal Institute of Technology (ETH) Zurich in 1969, by presenting him the highest rank of honours in FIP, the Freyssinet-Medal, which he received in 1982 at the Stockholm Congress. Max Birkenmaier retired in 1985 from the FIP and from active work. He enjoyed only 10 years of his retirement in good health; after a long time of suffering he died at the end of February 2002. A farsighted, wise and modest personality left us.

Hans Rudolf Mueller

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Steering committee and council meetings, Naples By invitation of the Italian national group the meetings were held in common on Friday 5 April 2002, followed by a separate meeting exclusively for members of Council on Saturday. About 30 participants attended the common meeting, representing 15 national groups (out of 39). In the Steering Committee there were six out of 10 commission chairmen and eight out of 10 elected members present. The most important items treated are briefly summarised in the following:

Next model code Several invited presentations animated the discussion. Safety and reliability of structures—a new safety approach was the title of the first one made by Ton Vrouwenfelder, the President of the Joint Committee for Structural Safety (JCSS). As a second invited speaker, Manfred Wicke, the Head of the Austrian Group, reported on experiences in up-grading and strengthening of concrete structures. Steen Rostam, chairman of fib Commission 5 Structural service life aspects, then gave the third invited contribution on Design for durability. Finally, Joost Walraven, as President of

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the fib, addressed new developments—how to deal with new high performance materials. A thorough discussion of all these presentations lead to the conclusion that the new Model Code should become a code for ‘producing’ (i.e. including both, design and construction) and maintaining good concrete structures (not: concrete!). Design and construction aspects need to be treated in one document. A draft list of contents should become available for the next meeting of the Steering Committee.

Commissions and task groups All chairmen and deputy-chairmen will be formally addressed by the secretariat, reminding them that according to the statutes their own and their commission’s members appointment ends after four years, in 2002. They will be asked to inquire among their membership about their interest to continue, and inform through the secretariat the Steering Committee whether or not they are personally prepared to stand for a second term of four years. A new Steering Committee will be

elected in the General Assembly in Osaka and shall propose to Council the chairmen and deputy-chairmen of all commissions for the period 2002–2006.

Congress and symposia Preparations for the Osaka congress are well on schedule. All commissions will have the opportunity to report extensively on their work, giving a certain priority to recently finalised work published as bulletins. Several countries will prepare printed national reports. The fib stand in Osaka will feature an interactive presentation of all selected entries, special mentions and winners of the 2002 Award for Outstanding Structures. Council finally accepted the proposal of the Italian Group to host the next fib Congress in 2006 in Naples. The PhD symposium in September in Munich (see calendar in this issue) will be very well attended. The next one (in 2004) was agreed to be held in Delft, and two years later in Zurich. The preparations for the 2003 fib symposium in Athens (see calendar in this issue) are progressing. For 2004 it is intended to have two fib symposia, one in France and one in India, and again two for 2005, in Hungary and Argentina. The next meeting of the fib Steering Committee and the Council will be held on 12 October 2002 in Osaka.








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consistent design and detailing tools like strut-and-tie models

fib, the already well advanced work was finished by a small editorial group. The purpose of the report is twofold: firstly, to give an overview of the issues relating to the management of concrete structures in general and, secondly, to supplement this with details on items concerned with assessment and remedial actions, as these are important technical parts of management and maintenance systems. The more general aspects of asset management are dealt with in chapter 1 which is mainly aimed at owners and decision-makers. Chapters 2 and 3 concern the information required for decision making in the assessment process and are aimed more at consultants and contractors. A review of remediation techniques is given in chapter 3 which is intended to assist in the selection of remedial actions rather than their execution. The report also includes some significant appendices regarding load testing, monitoring, fire and also special considerations related to seismic retrofitting. It is worthwhile also mentioning the work presented in Appendix 1 keywords, that should guide and encourage the various actors who are playing a role in this field, to use a common language

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New bulletins The fib Bulletins for the subscription year 2002 start with number 16 Design examples for the 1996 FIP recommendations Practical design of structural concrete; and number 17 Management, maintenance and strengthening of concrete structures. They have already been mailed to all corporate and individual subscribing members. A brief description is given in the following. Non-members may order these Bulletins or former publications, also from former CEB and FIP (before 1998), by simply following the instructions given on fib’s website http:// fib.epfl.ch/publications/.

Design examples (fib technical report) The 1996 FIP recommendations Practical design of structural concrete were finally published by the SETO in September 1999. They had been developed based on the 1990 CEBFIP Model Code. The main objective of this Bulletin is to demonstrate by practical examples the application of these recommendations, and especially to illustrate the use of strut-and-tie models for designing discontinuity regions in concrete structures. These examples represent a continuation of the 1990 FIP Handbook on Practical Design that had been based on the former (1984) version of the recommendations. Most of the examples are recently built existing structures. Although some of them may be considered as quite important, the chosen examples are by no means exceptional. The technical report does not deal with the discussion of aesthetic or general conceptual aspects. On the contrary, the main aim is to treat particular design aspects by selecting local regions of the chosen structures, that are then designed and detailed following the design principles and specifications proposed in the 1996 FIP recommendations mentioned above. The document is believed to be of interest to all engaged in the design of structural concrete. It hopefully supports the use of more








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Management, maintenance and strengthening of concrete structures (fib technical report) The report is the result of the work of the former FIP Commission 10 Management and strengthening of concrete structures that succeeded in 1995 to the former FIP Commission 10 Maintenance, operation and use. Close contacts had also been kept to the former CEB Commission V Operation and Use and in particular to its Task Group 5.4 Assessment, maintenance and repair. When in 1998 FIP merged with CEB to form the new association 115